1. Field of Invention
The present general inventive concept pertains to photovoltaic cell production, and more particularly, to a method of producing a high-efficiency, high yield photovoltaic cell.
2. Description of the Related Art
Photovoltaic cells, commonly known as solar cells, are known devices used for direct conversion of solar radiation into electrical energy. Generally, solar cells are fabricated on a semiconductor wafer or substrate using semiconductor processing techniques to form a p-n junction near a surface of the substrate. Solar radiation impinging on the surface of the substrate creates electron and hole pairs in the bulk of the substrate, which migrate to p-doped and n-doped regions in the substrate, thereby generating a voltage differential between the doped regions. The doped regions are coupled to a conductor on the solar cell to direct an electrical current from the cell to an external circuit coupled thereto. Commonly the n-type junction is on the surface based on phosphorous doping, and the p-type junction is in the body of the silicon wafer based on boron doping.
Use of solar cells in electrical energy production is attractive, in part due to the inexpensive nature of solar radiation used to fuel energy production in the solar cells. As the fuel costs associated with more conventional means of electrical energy production increase, demand for solar cells for electrical energy production also increases. However, previous methods for production of solar cells have proven to be labor intensive, energy intensive, and materials intensive, and such previous methods have often resulted in the production of solar cells having relatively low efficiency in electrical energy production. For example, in one known method of producing solar cells, a polycrystalline silicon material is used as the substrate. In this method, polycrystalline silicon wafers are obtained by first placing and packing essentially pure lumps of silicon in a crucible. The crucible is loaded into a vacuum furnace that has heating elements made of graphite. The vacuum furnace heats the silicon lumps, causing them to melt. Thereafter, the melted silicon is cooled to encourage the formation of a large silicon ingot defining polycrystalline crystal formations. The crucible is commonly constructed of fused silica and, because of the processing temperature of the vacuum furnace, the fused silica partially converts to crystobalite during the melting process within the vacuum furnace. Thus, the crucible is a single use and expensive tool. The melting process also produces a polycrystalline silicon ingot having a large outer volume contaminated by impurities reacted with the melted silicon. This outer volume is cut away and discarded following manufacture of the polycrystalline ingot. Thereafter, the somewhat more pure remaining inner portion of the ingot is thin-cut into discreet wafers using a wire saw in a diamond slurry, causing a very large additional loss of silicon, and also limiting the minimum thickness the wafer can be fabricated. The polycrystalline silicon wafers are then laminated to a conductive layer, thereby forming the solar cell as described above.
The time required to complete the heating-cooling cycle in the above-described method is often 45-60 hours. Thus, the time associated with performing the above-described heating-cooling cycle can result in significant delays in the production process. One factor in determining the length of the heating-cooling cycle is the time required to heat the silicon feedstock sufficiently to cause it to melt. As a general rule, the smaller the initial lumps of silicon to be melted, the faster the heating and melting cycle. However, it has been found that the process of diminution of the silicon lumps into silicon granules often smears undesirable contaminants onto the surface of the resulting silicon granules. Thus, the silicon lumps used are typically very coarse, having an average size of approximately thirty (30) millimeters. Use of such coarse materials helps to preserve purity in the melted silicon. However, the packing density of the above-discussed silicon lumps in the mold is approximately thirty-five (35) percent of perfect packing, which is significantly less than ideal. Consequently, heat is not conducted efficiently through such a feedstock, and additional heating time is required. Given that (1) the heating elements are on the outside of the crucible, (2) the silicon lumps have relatively little physical contact with one another, and (3) the silicon lumps often serve to “shadow” one another from thermal radiation very heavily; most of the heating occurs by thermal radiation that is accomplished in succession, wherein a relatively exposed silicon lump is heated, and that lump then radiates heat to one or more successive relatively unexposed lumps, i.e., one or more of the silicon lumps that are “shadowed.” In many instances, a partial pressure of argon gas is used to assist in transferring heat to the silicon feedstock.
The above-discussed process of melting the silicon material often results in undesirable random crystal structures present in the silicon ingot, contributing to a low efficiency in the resulting polycrystalline silicon wafer for converting sunlight into electrical energy. During the heating and subsequent cooling of the silicon, two impurities, i.e., silicon carbide (SiC) and dissolved oxygen complexes (including silicon-oxygen complexes), are produced in the silicon feedstock. These impurities cause a reduction in the yield of usable silicon crystal wafers that can be as high as approximately forty (40) percent. Also, these impurities cause defects in the crystal structure that reduce the efficiency and life of the resulting solar cell.
At least a few factors encourage the synthesis of these impurities. First, the high temperatures achieved in the furnace promote the oxidation of its graphite heating elements by reduction of the fused silica with which the graphite is in physical contact, thus creating a partial pressure of CO and CO2. Other components of the vacuum furnace may be composed of graphite as well, including the insulation material, and likewise, may too be oxidized. This oxidation-reduction reaction commonly yields two gases: carbon monoxide (CO) and carbon dioxide (CO2). These gases then react with the silicon feedstock in the mold to yield silicon carbide and dissolved-oxygen complexes. Second, although rebonded fused silica is a highly refractory substance, it is permeable by carbon oxide gases (e.g., CO and CO2). Thus, carbon oxide gases access the silicon feedstock by permeating the mold. Third, the packing density of the silicon lumps results in spaces that can be permeated and/or occupied by the carbon oxide gases. The surfaces of the silicon lumps that border these spaces serve as additional loci for the oxidation-reduction reaction that yields silicon carbide and dissolved-oxygen complexes.
Because of the process time of the vacuum furnace and the size of the melted silicon ingot, it is impractical to directly “dope” the melted silicon ingot to form the body of the n-type or p-type junction. Because of the limits of the doping technology, doping of the silicon is generally limited to using boron in the body of the silicon wafer to make the p-type junction and phosphorous at the surface of the silicon wafer to make the n-type junction. However, the methods of applying phosphorous to the surface of the silicon result in much larger coatings than are needed or can be achieved with boron. Finally, it must be acknowledged that vacuum furnaces generally do not create perfect vacuums, allowing atmospheric gases and potentially other gases to enter. Atmospheric gases include oxidizing agents that, as described previously, can result in the production of impurities.
A further yield loss is incurred by the sawing and slicing of the billet into wafers and subsequent tooling to attach the conductor, thereby forming the solar cell. Polycrystalline silicon is a relatively hard and brittle material, and thus, the operations of cutting the polycrystalline material and attaching the conductor are inherently difficult and labor intensive and result in a high mortality rate of the thin-cut silicon wafers due to fracture of the wafers during tooling and handling. In at least some instances, by the time the above-discussed contaminated outer layer is removed from the silicon ingot and the ingot is sliced down to silicon wafers having a thickness of approximately 150-200 microns, by the time the conductors are attached to the silicon wafers, and by the time resultant fractured wafers are discarded, the yield of usable thin-cut silicon wafers on starting silicon can be as low as 10-30%. In light of the above, the low yield of usable silicon wafer material and the high costs per unit of solar conversion efficiency associated with manufacture of solar cells using the above-discussed process have made use of solar cells manufactured by the above-discussed process for electrical energy production in the residential, commercial, and utility sectors impractical in many applications from an economical point of view without large subsidies from governments and the like.
U.S. Pat. No. 7,604,696 (“the '696 patent), which was issued to Carberry, describes one method of converting a silicon powder slurry into thinly molded solar grade silicon wafer. However, other known methods often lead to increased contamination of the resultant silicon wafers during the heating and cooling cycle. Accordingly, there is a continuing need for an improved method to produce a high-efficiency, solar cell.
If a silicon wafer body is uniformly doped at low levels with phosphorous, use of boron to dope the surface of the silicon wafer to establish a p-type junction results in greatly increased efficiency in converting photons to electrons by the resultant solar cell. However, prior art doping technology makes this type of uniform doping of phosphorous in a solar grade silicon wafer impractical in a commercial setting. Furthermore, while a silicon wafer of approximately 180 microns in thickness captures substantially all sunlight, a silicon wafer of approximately 40 microns still captures most sunlight, i.e., in excess of 96% of sunlight in normal conditions. The slight loss of light capture of the 40 micron wafer is more than compensated for in efficiency increases by decreased instances of recombination associated with a shorter path through the 40 micron silicon wafer. Thus, it is believed that a thinner wafer, perhaps 40 microns thick, would be optimal for use in solar cells. However, such a wafer cannot be made and handled by prior existing technology absent significant breakage of the wafer as discussed above.